1. Field of the Invention
The present invention relates to transmission devices, and more particularly, to a transmission device provided in a synchronous communication network to transmit signals.
2. Description of the Related Art
As a result of the recent demand for massive transmission of information, signals have come to be transmitted in larger volumes and at higher speeds. For subscriber-side interfaces in particular, those using Ethernet (registered trademark)—based technology have been developed, besides conventional DS1/DS3 techniques. Also, more and more diverse services are being incorporated in SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy) networks as backbone networks.
MSPP (Multi Service Provisioning Platform) has been devised in order that diverse services may be taken care of by a single platform. To efficiently provision the individual services, each card (unit) is provided with the necessary SONET/SDH STS/AU POH (Path Overhead byte) termination/generation function (PTE (Path Termination Equipment) function).
Meanwhile, semiconductor scaling techniques have also advanced, enabling a single unit to process a larger volume of signals. Accordingly, line error that occurs at the time of switching to a standby unit during the maintenance exerts an influence to a greater extent.
To avoid such an inconvenience, there has been an increasing demand for systems capable of switching units without entailing a momentary interruption.
FIG. 17 illustrates wirings of a conventional transmission device for transferring a frame timing signal and a multi-frame signal. The figure shows external interface PIUs (Plug-In Units) 101 to 104, STS cross-connect PIUs 105 and 106, VT cross-connect PIUs 107 and 108, a bus 109, and a CPU 110. Also, in the figure, W101 to W104 denote wirings for transferring an 8-kHz frame timing signal, and W105 to W107, indicated by dashed lines, denote wirings for transferring a 2-kHz multi-frame signal. W108 denotes wiring via which the STS cross-connect PIUs 105 and 106 synchronize their 8-kHz frame timing signals with each other.
Further, in FIG. 17, (W) and (P) signify Work and Protect, respectively. The illustrated transmission device is configured for SONET; in the case of SDH, each STS cross-connect PIU is replaced by an AU cross-connect PIU and each VT cross-connect PIU is replaced by a TU cross-connect PIU. In the following, explanation is directed to SONET but applies also to SDH.
The external interface PIUs 101 to 104 are interfaces for exchanging signals with external devices. The STS cross-connect PIUs 105 and 106 are each a PIU with STS-level cross-connect function, and the VT cross-connect PIUs 107 and 108 are each a PIU capable of VT pointer processing and having VT cross-connect function.
The external interface PIUs 101 to 104, the STS cross-connect PIUs 105 and 106 and the VT cross-connect PIUs 107 and 108 constitute a redundant configuration and are each set as an active (ACT) or standby (STBY) PIU. Active and standby PIUs carry out identical signal processing, and the signal from the active PIU is selected at the input of the next-stage PIU.
Let us suppose, for example, that the STS cross-connect PIU 105 and the VT cross-connect PIU 107 are active PIUs and that the STS cross-connect PIU 106 and the VT cross-connect PIU 108 are standby PIUs. In this case, the VT cross-connect PIUs 107 and 108 select the signal from the STS cross-connect PIU 105, and the STS cross-connect PIUs 105 and 106 select the signal from the VT cross-connect PIU 107. The external interface PIUs 101 to 104 select the signal from the STS cross-connect PIU 105.
In the transmission device, an 8-kHz frame timing signal is generated to synchronize frames within the device so that at the time of switching between the active and standby PIUs, the switching may be effected without causing a momentary interruption.
In the case of the transmission device shown in FIG. 17, the STS cross-connect PIUs 105 and 106 are each provided with a circuit for generating an 8-kHz frame timing signal, which is supplied via the wirings W101 to W104 to the respective PIUs. Also, the STS cross-connect PIUs 105 and 106 synchronize their 8-kHz frame timing signals with each other via the wiring W108.
Thus, even in the case where the STS cross-connect PIU is switched from the standby PIU 106 to the active PIU 105, for example, the switching can be effected without entailing a momentary interruption, because frame synchronization is achieved through the wiring W108. Also, since the VT cross-connect PIUs 107 and 108 are supplied with the same 8-kHz frame timing signal through the wiring W103 or W104, frames are synchronized, whereby switching between these two PIUs can be carried out without causing a momentary interruption.
The VT cross-connect PIUs 107 and 108 use, however, a 2-kHz multi-frame signal for VT pointer processing. Specifically, the VT cross-connect PIUs 107 and 108 individually divide the frequency of the 8-kHz frame timing signal to generate a 2-kHz multi-frame signal independently of each other. Thus, the multi-frame signals generated by the respective VT cross-connect PIUs 107 and 108 can become desynchronized with a probability of 3/4. If the active/standby switching is executed while synchroneity is lost, the primary signal suffers error.
To achieve synchroneity between the multi-frame signals of the VT cross-connect PIUs 107 and 108, the STS cross-connect PIU 105, 106 may divide the frequency of the 8-kHz frame timing signal and distribute the generated 2-kHz multi-frame signal to the VT cross-connect PIUs 107 and 108. For example, the 2-kHz multi-frame signal is distributed via the wiring W105 or W106 to the individual VT cross-connect PIUs 107 and 108.
Alternatively, the VT cross-connect PIUs 107 and 108 may exchange signal generation timings with each other to synchronize the multi-frame signals. For example, the multi-frame signal generation timings are exchanged via the wiring W107 to achieve synchroneity between the multi-frame signals.
The multi-frame signals of the VT cross-connect PIUs 107 and 108 are synchronized in the aforementioned manner, to prevent unwanted error from occurring at the time of active/standby switching.
All PIUs in the transmission device, including the STS cross-connect PIUs 105 and 106 and the VT cross-connect PIUs 107 and 108, are connected to the CPU (Central Processing Unit) 110 via the bus 109 for the purpose of various settings by firmware as will as other control actions. In FIG. 17, the connections of the bus 109 with the external interface PIUs 101 to 104 and the STS cross-connect PIUs 105 and 106 are not illustrated.
Meanwhile, there has been proposed a fault tolerant controller which includes a main system control part and a slave system control part and in which, in case of abnormality of the main system control part, the slave system control part is started to succeed the control mode of the main system control part based on the count value of a slave system counter, thereby to control the control object (see, e.g., Unexamined Japanese Patent Publication No. H11-134209).
In the conventional transmission device, however, exclusive wiring (in FIG. 17, the wirings W105 to W107) is needed to synchronize the timing signals (multi-frame signals) obtained by dividing the frequency of the reference signal (frame timing signal), giving rise to the problem that the wiring is correspondingly complicated and enlarged in scale.